In the fabrication of integrated circuits, lithography is used to generate pattern structures on the semiconductor and various materials for the construction of the desired circuit structures. A continuing demand in view of the ever increasing desire in the semiconductor industry for higher circuit density in microelectronic devices has prompted lithographic engineers to develop improved lithographic processes. Especially, a lithographic process can provide improved linewidth control. Many linewidth variations are due to focus uncertainty caused by numerous effects, such as resist thickness variations, bake non-uniformities, batch-to-batch resist sensitivity changes, non-flat wafers, lens field non-flatness etc. Therefore, to improve linewidth control one must either improve the focus window of the process or reduce the focus variations.
A variety of automated schemes for determining tool focus have been adopted. In state of the art exposure systems, there is an auto focus leveling sensor system to adjust the wafer in Z direction (perpendicular to resist surface) to achieve the best focus for resist exposures. Although the auto focus leveling sensor system will accurately place the wafers being exposed at the best focus location (or a predetermined offset from this position) by high-precision mechanical means, it is still susceptible to slight drifts in the position mechanism or changes in the required offset for various wafer stacks or various material sets (different photoresists, interlayers and/or underlayers). In addition, variations in imaging and process parameters may cause variations in the best focus position in the resist film, which in turn may cause variations in the dimensions of printed patterns. Therefore, the focus position adopted in the exposure system must be continually monitored to ensure that the linewidth of printed patterns is tightly controlled within an acceptable range.
The most direct method of achieving higher area density is to improve the resolution of circuit patterns in resist films. One way of improving the resolution in resist is to migrate to shorter wavelength from 365 nm to 248 nm, then to 193 and 157 nm, to go to extremely small wavelengths optical systems such as EUV (extreme ultraviolet), or to adopt non optical system such as E-beam. EUV lithography with exposure wavelengths below 20 nm allows the industry to print features beyond the diffraction limit of the current 193 nm lithography without resorting to the adoption of tricks using double or triple patterning. Focus variations can also be caused by diffraction effects due to the three-dimensional nature of the mask absorber features which are large compared to the wavelength of EUV. The three-dimensional features interact with the incident and reflected electromagnetic field and can cause focus variations on the wafer. These focus variations mean that it becomes even more important to have a metric for focus.
Some focus monitoring methods have been reported previously to determine focus variations during lithographic process. For example, Brunner et al., in U.S. Pat. No. 5,300,786, teach a focus monitoring method that can be used to accurately determine the best focus and focal plane in an optical projection system, but it requires the use of a specialized mask that cannot be integrated with product masks. The mask requires a 90 degree phase shift which is not a standard process. To fabricate 90 degree phase shift for EUV mask is very hard, likely involving offsetting the multilayer in Z direction, thus it is impractical to add this feature to a production mask. Although Sun et al. in a 2013 SPIE paper (Proc. of SPIE, Vol. 8679, pp. 86790T1-12, 2013) demonstrate an EUV mask having phase shift mask structure with 90.9° and 180° phase shifts for focus monitoring, they also disclose how difficult it is to build such phase-shifted targets. This is more evidence that EUV phase shift mask is too expensive and impractical for actual production of EUV masks.
In addition, Brunner et al., in U.S. Pat. No. 7,455,939, teach a method of making a process monitor grating pattern for use in a lithographic imaging system. The method measures focus in a single exposure, and uses sidewall angle of a resist measured by scatterometer to determine focus. In EUV lithography, extremely thin resists (˜30 nm) are used with rough sidewall, so the sidewall angle measurement is very hard if not impossible.